In stead of the formerly used vulcanized rubber as the main material, thermoplastic elastomers are now used frequently in automobile parts, domestic electric components, medical device components and sundry goods because of their excellent productivity. Examples of such cases include olefin elastomers comprising ethylene-propylene copolymer and polypropylene, polyurethane elastomers, soft polyvinyl chloride and the like.
Each of these molding materials, however, has disadvantages in terms of scratch resistance, flexibility, workability, economical efficiency and recycling ability. That is, olefin elastomer is relatively cheap and excellent in terms of weather resistance and heat resistance but is inferior to other materials in flexibility and scratch resistance. Polyurethane elastomer is possessed of excellent scratch resistance but has disadvantages in that it has poor flexibility and weather resistance and is expensive. Also, soft vinyl chloride is relatively cheap and excellent in terms of weather resistance and scratch resistance but has disadvantages in terms of flexibility at low temperature and recycling ability.
In addition, some elastomer compositions have been proposed in which hydrogenated derivatives of a vinyl aromatic compound-conjugate diene compound block copolymer (hereinafter referred to as a "hydrogenated block copolymer") is used. For example, each of JP-A-50-14742, JP-A-52-65551 and JP-A-58-206644 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a composition in which a hydrogenated block copolymer is blended with a rubber softening agent and an olefin resin. However, these compositions are also poor in scratch resistance similar to the case of olefin elastomers.
On the other hand, the air bag system is principally composed of a collision sensing device which detects collision of a high-speed movable body and an air bag apparatus, and the latter air bag apparatus is composed of an air bag, a gas generator which generates a gas to inflate the air bag, a housing cover which contains the air bag and gas generator and an adapter (retainer) which attaches the gas generator and housing cover. The air bag apparatus is attached to the front side of an occupant in a high-speed movable body, and, when the collision sensing device is activated by a collision, the gas is generated instantaneously from the gas generator and filled in the air bag which is folded and contained in a space formed by the surrounding gas generator, housing cover and retainer, and the housing cover is then unfolded by the pressure of gas filled in the air bag which is immediately released and inflated from the opening created by the unfolding toward the front side of the occupant, so that the occupant is confined to the seat and prevented from injuries by collision with the operating unit or instrument panel. In consequence, when the gas generator is activated by a collision or the like accident, the housing cover of air bag apparatus must release the bag instantaneously through its accurate unfolding without spreading splinters having a danger of injuring the occupant.
As such a housing cover of air bag apparatus, an air bag apparatus-housing cover formed from a urethane resin containing a reinforcing net mainly made of nylon has been proposed in JP-A-50-127336 or JP-A-55-110643.
Since a reinforcing nylon net is used, the just described air bag-housing cover is free from the problem of generating cracking on a part other than the opening portion and causing scattering of a part of the cover when the cover is unfolded, but has other problems such as a prolonged period of time necessary for incorporating the reinforcing net or displacement of the position of the reinforcing net at the time of molding, as well as a disadvantage of being poor in productivity.
Also, JP-A-202550 proposes a housing cover having a structure in which a surface layer made of a soft material having an A hardness according to JIS K6301 of from 30 to 70 and a core layer made of a hard resin having flexibility are integrally molded and a slit for cleavage use is arranged on the core layer. The housing cover of this proposal is desirable in terms that it has a certain degree of rigidity and gives the occupant proper feel, but it has a problem in that it requires two layer molding for the core layer and surface layer so that it requires an expensive double layer molding machine having two sets of injection mechanism. In addition, because of the poor surface anti-flaw ability (scratch resistance) of the surface layer soft material, it is necessary to coat the surface after the double layer molding, thus resulting in high cost.
JP-A-38996 proposes an air bag apparatus-housing cover formed from a thermoplastic elastomer composition mainly comprising a hydrogenated styrene-conjugate diene block copolymer having an A hardness according to JIS K6301 of from 60 to 85. The housing cover of this proposal is desirable in terms that it gives the occupant properly soft feel and adaptability is realized within a broad range of temperature, but it has a problem in that it is necessary to coat the surface after injection molding because of the poor anti-flaw ability (scratch resistance) of the surface, thus resulting in high cost.